I am currently having some difficulty with a card probability question (I guess combinatorics will haunt me for the whole life...). The question is to find the probability of getting a heart on the first draw and a club on the second draw while drawing 2 cards successively and without replacement from the standard poker deck. There are 2 approaches that I came with but they seem to produce different results.
Logically, I believe the answer to be $$ P(C|H)=\frac{\binom{13}{1}}{\binom{52}{1}}*\frac{\binom{13}{1}}{\binom{39}{1}}=\frac{1}{12}=0.083 $$ However, I can't formally prove that it is the right approach, so I tried using the conditional probability formula.
$$ P(C|H)=\frac{P(C\cap H)}{P(H)}=\frac{\binom{13}{1}*\binom{13}{1}}{\binom{52}{2}}/\frac{\binom{13}{1}}{\binom{52}{1}}=\frac{2}{51}=0.0392 $$
One(or even 2) of my answers are clearly wrong, so I would be glad if someone could tell me where my mistake is and how to fix one(or both) approaches.
The straightforward way to answer this question is to see that the probability of a heart on the first draw is $13/52$, or, equivalently, $1/4$.
Then there are $51$ cards left, so the probability of a spade the next time is $13/51$.
Multiply.
(I haven't addressed your question about what's wrong with your proposed approaches. You don't say why you think either might be right.)